1. Field of the Invention
The present invention relates generally to an improvement in a spark-ignition internal combustion engine, and more particularly to spark timing control technique for such an engine whose intake system function is variably controlled in accordance with engine operating conditions.
2. Description of the Prior Art
In connection with spark-ignition internal combustion engines, to appropriately control combustion efficiency of the engine in accordance with engine loads, it has been proposed that an intake air pipe is branched off at its intermediate section into first and second parallel intake passage portions leading to engine combustion chambers, in which intake air is introduced through both the branch intake passage portions during a high load engine operation requiring much intake air while through only the first branch intake passage during a low load engine operation requiring less intake air.
This deals with drawbacks in which, in case intake air is introduced through a single intake pipe throughout all engine load operating ranges, the flow rate of intake air unavoidably lowers to deteriorate combustion during the low engine load operation upon the cross-sectional area of the single intake pipe being decided greatly taking account of power output characteristics at high engine load. In other words, with the engine provided with the first and second branch intake passage portion, intake air flow resistance decreases to obtain high engine power output during the high engine load operation in which intake air flows through both the first and second intake passage portions, whereas the flow rate of intake air increases to improve combustion during the low engine load operation in which intake air flows through only the first intake passage portion.
In the thus arranged engine in which combustion state in the combustion chamber changes depending upon condition of an intake system, spark timing is changed in accordance with the opened or closed state of the second intake passage portion since optimum spark timing changes depending on combustion state. More specifically, spark timing is set to be advanced during high load engine operation relative to during the low load engine operation, because intake air flow rate decreases to lower combustion speed thereby to lower engine power output during the high engine load operation in which both the first and second intake passage portions are used. In other words, this avoids abnormal combustion such as engine knock by retarding spark timing in accordance with an increase in intake air flow rate during the low load engine operation closing the second intake passage portion, which spark timing retardation is made relative to the high load engine opration.
Opening and closing the above-mentioned second intake passage portion is carried out by a blocking valve movably disposed in the passage portion. The blocking valve is usually operated by a fluid-pressure operated actuator such as a diaphragm actuator. Such an actuator is disadvantageous in a point of being low in operating speed or response, though it is advantageous in points of being low in and high in responsibility. This provides the following problems: Correction of spark timing and opening and closing actions of the blocking valve are carried out in accordance with signal representative of engine load condition such as intake air amount, in which the spark timing is changed onto an advanced side and simultaneously the valve is opened when engine load increases to some extent from a low load level at which the valve is closed. However, although intake air amount and fuel amount increase at this time, a considerable time is required from initiation to completion of opening operation of the blocking valve owing to the above-mentioned low response. Accordingly, during this time, rich air-fuel mixture flows at a high speed into the combustion chamber and additionally spark timing has been already changed onto the advanced side. This tends to raise engine knock.